Add a FAQ on how users can check their version of Tor Browser on all platforms

As part of our user support work and for troubleshooting in general, we fairly regularly ask users about the version of Tor Browser they are using. I think we should add some steps on how users can check their version of Tor Browser.

For Tor Browser on desktop we can create a entry in https://support.torproject.org/tbb/ and for Tor Browser for Android in https://support.torproject.org/tormobile/

/cc @nina